One such machine is described in UK patent specification No. 1452969. In this machine the adhesive supply means comprises a ratchet-operated gear pump for supplying adhesive through the creaser foot, the ratchet being carried by an oscillating rod arrangement, said rod arrangement being connected to the workpiece feeding means, so that the speed of oscillation is determined by the speed of rotation of the main drive shaft. The connection with the workpiece feeding means, furthermore, is such that any variation of the rate at which the workpiece is fed, under the control of the workpiece feed rate varying means, is effective to vary also the amplitude of the oscillation of the rod arrangement, and thus the distance through which the gear pump is rotated under the action of the ratchet.
It will be appreciated that, by using a ratchet-operated gear pump, the supply of adhesive is necessarily intermittent and can take place only during certain parts of each cycle of the machine (i.e. of each rotation of the main drive shaft). While admittedly the workpiece feeding means conventionally also is operated intermittently, nevertheless problems may arise when adhesive is applied to the workpiece marginal portion in a series of blobs, rather than in the form of a continuous ribbon.
One problem frequently encountered in thermo-cementing and folding machines, furthermore, is the tendency for adhesive to "drool" from the creaser foot during the period between successive operations. Various solutions to this problem have been attempted; for example, in the machine described in the aforementioned Patent Specification, it is proposed to provide an on-off valve associated with the gear pump outlet, this valve being operated by the oscillating rod arrangement, through a Bowden cable. In machines of other types, however, one solution to the problem of drool has been to provide a so-called "suck back" arrangement, whereby adhesive is drawn away from the outlet port; one such arrangement, utilised when the adhesive is supplied in rod form, operates to withdraw the leading end of the solid rod in a direction away from the melt chamber so that back-suction is applied to the molten adhesive to draw it away from the exit nozzle or port.
It will of course be appreciated that, in the machine described in the aforementioned Patent Specification, because of the direct connection between the gear pump drive and the workpiece feeding means, such a suck back arrangement is not possible.
Also where a suck back arrangement is provided, it is, it will be appreciated, also at least highly desirable, if not necessary, to feed the adhesive at a fast forward rate in order to ensure that the supply of adhesive is immediately available when required at the start of the next operation. Where the arrangement for retracting the adhesive in rod form is utilised as set out above, the rod is then advanced through the same distance at the start of the next operation to provide this forward "spurt". Conventionally this is achieved by the rod feed mechanism itself being bodily moved towards and away from the melt chamber. However, in practice, it has been found that it is frequently not adequate merely to advance the rod through the distance through which it has been retracted, but that rather it would be desirable to advance the rod feed mechanism through a greater distance than that through which it had previously been retracted.
It will of course be appreciated that where, again, the adhesive supply is controlled by a gear pump directly connected to the workpiece feeding means, the possibility for a "fast forward" adhesive supply over a short period at the start of an operating cycle is not readily attainable.